The present invention generally relates to thermal printers, and more specifically relates to a thermal printer which uses a sacrificial member between a print head and the web on which the printer prints in order to reduce wear on the print head during the printing process. As used throughout this application, the term xe2x80x9cthermal printerxe2x80x9d shall mean thermal transfer printer as well as direct thermal printer.
Direct thermal printers are well known in the prior art. In such printers, a web of paper or film having a thermally sensitive coating is interposed between a driven platen roller and a thermal print head having a line of selectively energized heating elements. To print onto the web, an electrical pulse is applied to a selected set of the heating elements, and a localized chemical reaction occurs at corresponding points in the thermally sensitive coating on the web which results in the formation of visible dots on the web.
After a line of dots is printed, the web is advanced to locate an adjacent location of the web over the print head heating elements, and the selecting and heating process is repeated to print an adjacent line of dots on the web. This process is repeated in order to print complete lines of text or graphics on the web.
The heating elements of the print head which are selectively energized during the printing process are typically covered with a protective ceramic overcoat. Webs which are used to print images thereon also typically have thermally sensitive coatings. During printing, the web moves across the print head; hence, the coatings on the web and print head rub against each other. The rubbing of the web on the print head during printing causes abrasion of the overcoat on the print head, and this is a common mode of failure and a limitation on print head life. Additionally, because the web contacts the print head, it has been required to use expensive print media, specifically print media with expensive coatings in order to limit the wear on the print head.
In some printing applications, reactive components or metallic ions are used to produce an image on the thermally sensitive coating on the web. This causes the print head to be exposed to the reactive components or metallic ions. Unfortunately, the reactive components which are used in some printing applications can be corrosive to the print head. Additionally, the print head can become contaminated as a result of being exposed to metallic ions.
Because the print head of a thermal printer is subject to so much wear and exposure during the printing process, the print head is often considered to be an expendable maintenance item, despite the fact that the print head is relatively costly.
Thermal transfer printers are also well known in the prior art. In these printers, a nonsensitized web is customarily used and a transfer ribbon is interposed between the print head and the web having a coating of wax or resin which is selectively melted and thereby transferred to or chemically reacted with the web. This allows nonsensitized webs to be imaged and provides for a wide range of materials that can be used to form the image. The transfer ribbon can similarly expose the print head to reactive components or metallic ions, resulting in reduced print head life unless expensive back coatings are applied to the transfer ribbon to reduce the wear.
While the preferred embodiment is disclosed in terms of a direct thermal printer, the subject invention is equally adapted to thermal transfer printers, in which case, the sacrificial member is disposed between the print head and the transfer ribbon.
A general object of an embodiment of the present invention is to provide a thermal printer that positions a sacrificial member between a print head and the web in order to reduce wear on the print head.
Another object of an embodiment of the present invention is to sacrifice a sacrificial member, such as a belt or web of thermally conductive material, in lieu of or in addition to the print head overcoat by interposing the sacrificial member between the web and the print head.
Briefly, and in accordance with at least one of the foregoing objects, an embodiment of the present invention provides a thermal printer that includes a sacrificial member disposed between the thermal print head and the web. The thermal printer holds the sacrificial member generally under tension, and the sacrificial member contacts the thermal print head and generally prevents the web from directly engaging the thermal print head.
Preferably, the thermal printer includes a platen roller, and the sacrificial member and web are disposed between the thermal print head and the platen roller, however, it should be noted that thermal and thermal transfer printing can be applied to webs that are held against the print head by web tension alone, that the claimed sacrificial member can be used with such structures, and that such structures are intended to lie within the scope of the appended claims. It should also be noted that the preferred embodiment is disclosed in terms of belt drives from a common motor, but that equivalent structures having gear drives or independent motors and drives for the web and the sacrificial member are intended to lie within the scope of the appended claims.
The sacrificial member may take one of many different forms. Regardless of the form the sacrificial member takes, by providing a sacrificial member between the print head and web, wear and exposure of the print head is reduced, thereby prolonging the life of the print head.
The thermal printer may take the form of a thermal transfer printer, in which case a thermal transfer ribbon is disposed between the sacrificial member and the web.
Another embodiment of the present invention provides a method of thermal printing wherein the steps include interposing a sacrificial member between a thermal print head and a web, and energizing the thermal print head to heat the web through the sacrificial member.